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Something I can never understand is that where the cosmic background radiation spreads?

If I know well, the cosmic background radiation is actually the light of the Big Bang. If it happened exactly in the same time, it must have spread into the theoretic center of the universe. Which would mean that it already reached every parts of it, in case if it happened in the same time with the Big Bang.

It be possible that it spreads to this direction, but universe expands faster. In this case, the radiation approaches but also moves off - just like everything in the universe. But it's possible only if the universe expands faster than the speed of light. Is this possible?

If it would spread to the direction of the "edge" of the universe, we shouldn't be able to know about its existence, because it would never reach us.

Also, these theories are true with an important conclusion: universe has a beginning in time, which means that once upon a time, it started to expand - consequently it must have a size limit. The reason is that universe has 4 dimensions: length, width, height, and time. One of them (time) is not infinite, so none of the others can be infinite.

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A few points not worthy of an answer: space can expand faster than the speed of light, there is no theoretical center of the universe, your argument that since the universe is finite in time it must be finite in space would only be correct if the universe started out finite in space and it may have been infinite at the start of the big bang. – Brandon Enright Jun 1 '13 at 16:57
Just to be clear (w.r.t CMB as "light of the big bang" and coming from "exactly ... the same time"), the CMB is light as it was after matter-radiation decoupling, during the "recombination" epoch. I.e., this is when the rate of expansion of the universe exceeds the rate of Compton scattering. – sujeet Jun 1 '13 at 18:18
I know that I may be wrong at some points. But that's why I ask: to correct me if I'm wrong. – Zoltán Schmidt Jun 2 '13 at 13:24
up vote 4 down vote accepted

The CMB doesn't come from one place; it comes from everywhere. It also doesn't go in one direction; it goes in every direction. And it didn't happen all at once, but it happened at roughly the same time everywhere. In particular, it's not the light of the Big Bang, but the light of a time roughly 378,000 years after the Big Bang.

The idea is that the universe started out as a really hot soup of ions (where the atomic nuclei and electrons wouldn't stay together because they had too much energy). Photons couldn't get very far before being absorbed or scattered by these ions and electrons. But once the universe expanded and cooled a little more, neutral atoms could form. And atoms are far less efficient at absorbing and scattering photons than ions are. So any photon anywhere in the universe was suddenly unlikely to get scattered and absorbed. (Except for absorption by unusually dense clumps of matter like stars and big dust clouds, but we know that most of the universe is empty of those things.)

So the photons that had been bouncing around off of ions and electrons were suddenly free to roam the universe without interruption. And that's what they've been doing for the last 13.8 billion years or so, until some have reached our detectors. You can imagine that basically everywhere in the universe gave off the CMB light in basically every direction. It "spread out" with the rest of the universe, and across the universe, and just everywhere.

The CMB photons we detect now did come from a sphere (basically) and did come in a certain direction to get to us. But there's nothing special about us. We could move our detectors to a different place in the universe, and we'd see CMB photons that just came from a different sphere, in different directions.

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The big bang is completely centerless.

The right way to picture where the CMB is coming "from" is to imagine that, long ago, the universe was as hot as the surface of the sun. This means that the universe was filled with a super-hot plasma that looked much like the surface of the sun today. Then, after enough expansion, the gas cooled off, and the plasma de-ionized into a normal gaseous mixture of H and He. This happened everywhere all at once. This is called the time of recombination.

But, as we know from special relativity, it takes a finite amount of time for information to travel to us. So, for things 186,000 miles away, you could still see the super hot plasma 1 s after the point of recombination, and so on and so on. Since farther away objects are expanding away from us at faster speeds${}^{*}$, they therefore redshift, and so, the light from this plasma gets redder and dimmer over time, until, 13.6 billion years later, it has become the cosmic microwave background.

${}^{*}$And, commenters, I'm really not interested in hearing about your lecture about how expansion velocities aren't real. There is a coordinate system where the 3-d spatial geometry is static, and this is a perfectly valid interpretation, which is probably more suited to our Earth-bound units anyway, considering that they don't expand with the universe. Comoving coordinates aren't the only way to interpret cosmology.

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